Langmuir 1999, 15, 1591-1593
A Fourier Transform Raman Study of the Effect of Si/Al Ratio on Phase Transitions in Zeolite ZSM-5 Induced by Adsorption of p-Dichlorobenzene Yining Huang* and Ping Qiu Department of Chemistry, the University of Western Ontario, London, Ontario, Canada N6A 5B7
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vials were then sealed and placed in an oven at 150 °C for 3 h to uniformly disperse the sorbate molecules throughout the sample. All Raman spectra were recorded at room temperature on a Bruker RFS-100 FT-Raman spectrometer equipped with a Nd3+:YAG laser operating at 1064.1 nm and a liquid nitrogen cooled Ge detector. The laser power was typically 80 mW at the sample. The resolution was 2 cm-1. The powder XRD spectra were obtained on a Philips PW1050 powder diffractometer with Cu KR radiation.
Results and Discussion Received July 21, 1998. In Final Form: December 1, 1998
Introduction Many zeolites undergo structural changes induced by sorbed organic molecules. Phase transitions of this type are very important in the applications of these materials as sorbents and catalysts. In the past, X-ray diffraction (XRD) and 29Si MAS NMR have been extensively utilized to study the sorbate-induced structural changes.1 These techniques are mainly focused on the host framework itself. Recently, we have demonstrated that Fourier Transform (FT) Raman spectroscopy is a useful tool for the investigation of sorbate-induced phase changes in zeolites via the monitoring of the guest molecules.2 For the p-xylene/ZSM-5 system, we have shown that the spectral parameters of guest molecules such as band frequency, splitting, and line width are very sensitive to structural transitions in the host framework. In the present investigation, we have investigated the effect of adsorption of p-dichlorobenzene (pdcb) on the framework structure of ZSM-5. The reason for choosing this system is that there are good NMR and XRD data3 available in the literature against which to further check the viability and reliability of the Raman approach. However, in previous studies of pdcb/ZSM-5 complexes, all the ZSM-5 samples employed have relatively high Si/Al ratios. Since the phase transition behavior of ZSM-5 may depend on the Si/Al ratio,4 we examined host-guest interactions between pdcb and ZSM-5 with a wide range of Si/Al ratios (25, 199, 335 and silicalite-1). The results show that the FT-Raman method can provide unique information complementary to that obtained by XRD methods since this technique is sensitive to the short-range ordering and local structure. Experimental Section The zeolite samples were obtained from Chemie Uetikon AG and Union Carbide Corp. 1,4-Dichlorobenzene (99+%) was obtained from Aldrich Chemical Co. and used without further purification. Zeolite samples loaded with p-dichlorobenzene were prepared by adding precisely measured amounts of pdcb to weighed aliquots of freshly calcined ZSM-5 in glass vials. The * Corresponding author. (1) (a) Fyfe, C. A.; Mueller, K. T.; Kokotailo, G. T. In NMR Techniques in Catalysis; Bell, A. T., Pines, A. Eds.; Marcel Dekker, Inc.: New York, 1994; pp 11-67 and the references therein. (b) Gies, H. In Advanced Zeolite Science and Applications; Jansen, J. C., Stocker, M., Karge, H. G., Eds.; Weitkamp, J. Eds.; Elsevier: Amsterdam, 1994; pp 295-327 and the references therein. (2) Huang, Y. J. Am. Chem. Soc. 1996, 118, 7233-7234. (3) (a) van Koningsveld, H.; Jansen, J. C.; de Man, A. J. M. Acta Crystallogr. 1996, B52, 131-139. (b) van Koningsveld, H.; Jansen, J. C.; van Bekkum, H. Acta Crystallogr. 1996, B52, 140-144. (c) Gies, H.; Marler, B.; Fyfe, C. A.; Kokotailo, G. T.; Feng, Y.; Cox, D. E. J. Phys. Chem. Solids 1991, 52, 1235-1241. (d) Mentzen, B. F.; SacerdotePeronnet, M. Mater. Res. Bull. 1993, 28, 1161-1168. (4) (a) Hay, D. G.; Jaeger, H. J. Chem. Soc., Chem. Commun. 1984, 1433. (b) Vincent, R.; Man, P.; Fraissard, J. Catal. Lett. 1990, 4, 75-84.
pdcb/ZSM-5 (Si/Al ) 335). The structure of calcined ZSM-5 with a Si/Al ratio of 300 is monoclinic (P21/n).5 Recent single-crystal X-ray diffraction studies have suggested that the structure of ZSM-5 (Si/Al ) 300) with a loading of 2.6 pdcb molecules/unit cell (u.c.) (low-loaded form) is orthorhombic, Pnma,3a while the high-loaded form (8 molecules/u.c.) is also orthorhombic but has a different space group (P212121).3b FT-Raman spectra of pdcb adsorbed on ZSM-5 were measured as a function of loading from 1.5 to 8 molecules/u.c. Assignments of the vibrational modes of pdcb were based on those previously reported.6 To assist in interpreting the data, the spectra of pure pdcb solid and pdbc in a CCl4 solution were also recorded. Within the range of 1.5-3 molecules/u.c., the spectra of pdcb were independent of the loading. This indicates that the additional pdcb molecules access identical sites inside the framework, presumably at the intersections of the straight and sinusoidal channels. Further, it suggests that the structure of ZSM-5 remains unchanged throughout this loading range. The spectra of pdcb with low loadings are similar to that of a free molecule in solution but differ slightly from that of pure solid. For example, in the spectrum of pure pdcb solid, the C-C stretching mode (ν2) was observed as a doublet (a strong band center at 1572 cm-1 with an obvious shoulder at 1575 cm-1). The doublet is due to the correlation coupling between the molecules within the unit cell. However, similar to the solution spectrum, this mode appeared as a single band positioned at 1577 cm-1 in pdcb/ZSM-5 complex as a result of the pdcb solid “dissolved” in zeolite lattice, removing the intermolecular interactions between pdcb molecules. Once inside the zeolite channels, ν1 and ν11 (two C-H stretching modes6a) observed at 3072 cm-1 with a prominent shoulder at 3064 cm-1 in the spectrum of pure pdcb shifted by 5 and 6 wavenumbers to 3077 and 3070 cm-1, respectively. The changes in frequency are the result of the close match between the channel size and molecular dimension. The “tight fit” of the pdcb within the framework may cause the perturbation of the ring C-H stretching vibrations. Thus, the observed shift toward higher energies may be attributed to the restriction of the C-H stretching motions from the surrounding framework (a similar situation has been reported previously for template molecules trapped inside zeolites7). This is also consistent with the finding of a single-crystal X-ray diffraction study3a that the straight channel pore in low-loaded phase is distorted upon adsorption of the pdcb, resulting from the interactions between the guest molecule and framework. Interestingly, (5) van Koningsveld, H.; Jansen, J. C.; van Bekkum, H. Zeolites 1990, 10, 235-242. (6) (a) Green, J. H. S. Spectrochim. Acta 1970, 26A, 1503-1513. (b) Suzuki, M.; Ito, M. Spectrochim. Acta 1969, 25A, 1017-1021. (c) Stojiljkovic, A.; Whiffen, D. H. Spectrochim. Acta 1958, 12, 47-56. (7) Dutta, P. K. J. Inclusion Phenom. Mol. Recognit. Chem. 1995, 21, 215-237.
10.1021/la9809168 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/22/1999
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Figure 1. FT-Raman spectra of pdcb adsorbed in ZSM-5 (Si/ Al ) 335) at the loadings of (top) 8 and (bottom) 3 molecules/ u.c. in the regions of 3200-3000 cm-1, 1600-1550 cm-1, and 1130-1080 cm-1.
the frequency of ν6, the C-Cl stretching vibration at 329 cm-1 was not affected at all by the adsorption, implying that the long axis of the pdcb is oriented along the channel axis and, consequently, the C-Cl stretching vibration is not restricted by the framework. When the loading was increased to 4 molecules/u.c. the spectrum of adsorbed pdcb started showing distinct changes that were complete at a coverage of 6 molecules/ u.c. These results indicate that the zeolitic host framework undergoes a phase transformation in the 4-5 molecules/ u.c. loading range and the transition is complete at a loading of 6 molecules/u.c. Upon transition, several bands of pdcb adsorbed in zeolite split into two components. Figure 1 shows the splitting in the C-H stretching, C-C stretching, and C-H bending regions. The observed splitting in the high-loaded phase indicates the existence of two crystallographically nonequivalent pdcb molecules. For each observed doublet, the low-frequency component is assigned to the pdcb molecules located in the intersections while the high-frequency component to the second independent molecules positioned in sinusoidal channels. This assignment is based on the single-crystal X-ray diffraction data which suggest that the pdcb molecules in the sinusoidal channels are more tightly packed than pdcb at the intersections.3b The “tighter fit” of pdcb molecules in sinusoidal channels results in the shift of vibrational frequencies of several stretching and bending modes toward higher energies. The change in stretching frequency can be understood by using the argument of Dutta,7 that the expansion of molecular volume during the stretching vibration is resisted by the tight surrounding framework, resulting in a frequency shift toward higher energies. For the bending modes, it is believed that the stronger confinement restrictions on the encaged pdcb molecules imposed by the sinusoidal channels cause subtle changes in the bond angles from their average values in a free molecule. This change will increase slightly the restoring potentials and, at the same time, the energies of the bending modes. At the transition, several vibrational modes exhibited large shifts to higher energies. Figure 2A illustrates a sudden increase in the frequency of ν10, the C-Cl outof-plane bending mode at a loading of 5 molecules/u.c. This mode appeared as a single band at 294 and 301 cm-1 in the low- and high-loaded phases, respectively. This frequency shift in high-loaded phase is presumably due to the pdcb molecules on average having a more restricted environment for the C-Cl out-of-plane bending vibration, resulting in a higher barrier to the motion. Interestingly, these two peaks coexist in the spectrum of each sample with intermediate loading (4 and 5 molecules/u.c.) (Figure
Notes
Figure 2. (A) Plot of the frequency of the C-Cl bending mode of pdcb adsorbed in ZSM-5 as a function of the loading. (B) FT-Raman spectra of pdcb adsorbed in ZSM-5 in the region 320-260 cm-1.
Figure 3. Plot of the full width at half-height in cm-1 of (A) a Raman mode at 747 cm-1 of pdcb (B) the most intense Raman band of the zeolite framework as a function of the loading.
2B). The 301 cm-1 band gained intensity with increasing loading at the expense of the 294 cm-1 band, suggesting that both low- and high-loaded phases of ZSM-5 are simultaneously present in different proportions. The transition from the low- to high-loaded phase was also accompanied by the changes in the line width. Figure 3A shows the effect of loading on the line width of ν5, a ring deformation mode. The vibrational modes of the zeolitic framework also exhibited some interesting changes. For example, the most intense Raman band of the zeolitic framework appeared as a broad envelope centered at about 370 cm-1 in the low-loaded phase. Upon transition, the width of this profile decreased dramatically, from 56 cm-1 for the low-loaded phase to 32 cm-1 for the high loaded phase (Figure 3B). These results imply that the phase transition has taken place at a loading between 4 and 6 molecules/u.c. To confirm that the observed changes in the Raman spectra of the guest molecule are indeed due to the structural changes in the host framework, powder XRD patterns were also obtained. The differences in powder patterns among the ZSM-5 samples with the loading of 0, 2.5, and 7 molecules/u.c. (Figure 4A) correspond to three different phases of ZSM-5.8 Thus, the changes in XRD patterns are in agreement with the Raman data. The Raman spectra of pdcb adsorbed in ZSM-5 (Si/Al ) 199) and silicalite-1 at various loadings are almost identical to those discussed (8) (a) Fyfe, C. A.; Kennedy, G. J.; De Schutter, C. T.; Kokotailo, G. T. J. Chem. Soc., Chem. Commun. 1984, 541-542. (b) Wu, E. L.; Lawton, S. L.; Olson, D. H.; Rohrman, A. C., Jr. Kokotailo, G. T. J. Phys. Chem. 1979, 83, 2777-2781. (c) Long, Y.; Sun, Y.; Zeng, H.; Gao, Z.; Wu, T.; Wang, L. J. Inclusion Phenom. Mol. Recognit. Chem. 1997, 28, 1-15.
Notes
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Figure 5. Comparison of the Raman spectra of pdcb adsorbed in ZSM-5 with different Si/Al ratios and loadings: (A) Si/Al ) 335 at a loading of 6 mol/u.c.; (B) Si/Al ) 335 at a loading of 5 mol/u.c.; (C) Si/Al ) 25 at a loading of 8 mol/u.c.; (D) Si/Al ) 25 at a loading of 2 mol/u.c. Figure 4. The powder XRD patterns of pdcb/ZSM-5 with Si/Al ratios of (A) 335 and (B) 25 in the range 2θ ) 22-25°.
above, implying that the phase transition behavior of these materials is similar to that of ZSM-5 with a Si/Al of 335. pdcb/ZSM-5 (Si/Al ) 25). No study has been performed on the pdcb/ZSM-5 complex with such a low Si/Al ratio. Unlike the systems discussed above, the powder XRD patterns of pdcb/ZSM-5 complexes with the loadings of 0, 2.5, and 7 molecules/u.c. look almost identical (Figure 4B). The XRD patterns of unloaded and low-loaded samples are expected to be the same since the calcined and unloaded ZSM-5 with such a low Si/Al ratio is already in its orthorhombic phase at ambient conditions.4a However, the similarity of the XRD patterns between low- and highloaded samples seems to suggest that adsorption of pdcb in ZSM-5 does not induce any structural change in the zeolitic framework. FT-Raman spectra of pdcb adsorbed in ZSM-5 (Si/Al ) 25) were independent of the coverage within the loading range of 1.5-6 molecule/u.c., suggesting that there is no structural change in the framework. These spectra are also identical to those of pdcb in the low-loaded phase of ZSM-5 (Si/Al ) 335). The noticeable changes did not take place until very high loading levels (7 and 8 molecules/u.c.) were reached. The changes are the same as those observed for pdcb/ZSM-5 complexes with high Si/Al ratios but occur to a much lesser degree for a given loading. Careful inspection of the data reveals that the degree of the changes in the spectrum of pdcb/ZSM-5 (Si/ Al ) 25) at the maximum loading (8 molecules/u.c.) is
only comparable to that in pdcb/ZSM-5 (Si/Al )335) at a loading of 5 molecules/u.c (Figure 5). As discussed earlier, for ZSM-5 with Si/Al ) 335, incorporation of 5 pdcb molecules into the zeolite yielded an intermediate structure with both low- and high-loaded phases coexisting simultaneously. However, for ZSM-5 with Si/Al ) 25 to achieve the same effect much higher loading levels (7 and 8 molecules/u.c.) are needed. Thus, our results show that incorporating 8 pdcb to the ZSM-5 with Si/Al ) 25 can only induce an incomplete structural change in the zeolitic framework and this early change in framework is not detected by powder XRD. In summary, the present work further demonstrates the usefulness of FT-Raman spectroscopy in studies of sorbate-induced phase transition in zeolitic systems. For pdcb/ZSM-5 with Si/Al ratios being 199, 335, and ∞, the transition between two orthorhombic phases starts taking place at a loading of 4 molecules/u.c. and the transformation is complete at a loading of 6 molecules/u.c. For the material with Si/Al ) 25, the onset of the transition occurs at a much higher loading (7 molecules/u.c.). At the maximum loading (8 molecules/u.c.), the transition is still incomplete and two phases coexist. Acknowledgment. Y.H. acknowledges the financial support from the Natural Science and Engineering Research Council of Canada. We also thank Dr. R. T. Oakley for access to an FT-Raman spectrometer. LA9809168
